1. Field of the Invention
The present invention relates to an electric double layer capacitor and a method of manufacturing the same, and more particularly, to an electric double layer capacitor (EDLC) allowing for an increase in a surge voltage and an operating voltage and having a high energy storage density and a method of manufacturing the same.
2. Description of the Related Art
An electric double layer capacitor (EDLC) is an energy storage medium in which two electrodes of an anode and a cathode are arranged to face each other with a separator interposed therebetween so that a pair of charge layers (electric double layers) having different signs can be generated on the facing surfaces of the electrodes.
An EDLC is mainly used as an auxiliary power supply, an IC backup power supply or the like for a variety of electrical and electronic devices. In recent years, the EDLC has been widely used for applications including a toy, an industrial power supply, an uninterrupted power supply (UPS), solar energy storage, HEV/EV sub power, and the like.
An EDLC is generally manufactured by accommodating a unit cell in a case and then filling the case with an electrolyte. Here, the unit cell is constructed by alternately stacking electrodes and sheets of a separator.
Typically, in order to establish a proper voltage and capacitance required for an EDLC, two or more unit cells are connected in series and in parallel to form the EDLC.
A pair of electrodes have a positive polarity (+) or a negative polarity (−) determined according to the sign of external electricity applied thereto. Terminals to which external electricity are applied are drawn from the pair of electrodes.
In the pair of electrodes, positive (+) charges and negative (−) charges are polarized, and accordingly, two charge layers (electric double layers) are formed in a single unit cell.
In a conventional unit cell, however, a surge voltage is low, i.e., less than 3.0V, and an operating voltage is also low, i.e., 2.3V to 2.7V. Therefore, there is a problem in that two or more unit cells have to be connected in series in order to establish an operating voltage applicable to electronic products.
An EDLC has the area of facing surfaces (specific surface area) increased with the use of activated carbon electrodes, and capacitance improved with the use of an electrolyte. Increasing the area of the facing surfaces of the electrodes may increase capacitance.
Furthermore, the EDLC, which is an energy storage medium enabling instant charge/discharge, has a superior output characteristic to that of a battery but has a low voltage per unit cell since its voltage gradually drops simultaneously with discharge. Therefore, the EDLC has an energy storage density smaller than that of the battery. Accordingly, the EDLC has been generally used for an auxiliary power supply for the output of a battery, and an auxiliary power supply for other electrical and electronic devices.
Most of electronic products including ICs and backup power supply products require an operating voltage of 1.8V or greater, preferably a wide voltage range of 3V to 48V. Accordingly, in order for an EDLC to be used for these products, two or more unit cells are serially connected to increase the operating voltage.
However, in the case where two or more unit cells are serially connected to increase the operating voltage of a capacitor, there is another problem in that a balance problem between the unit cells, which inevitably occurs, should be solved. Specifically, there is a need for a voltage balance protection circuit such as a resistor, a diode and another IC so that the overall operating voltage of the capacitor is not concentrated on a single unit cell, in consideration of the capacitance of the unit cell, equivalent serial resistance (ESR), a leakage current, and the like.
Meanwhile, in an energy storage medium, a unique value is used to indicate the amount of energy that can be stored therein. In the case of a battery, 1 AH (storage capacity capable of supporting the use of a current of 1 A for an hour) is used to indicate the amount of energy, since the battery has a stable voltage range.
In the case of an EDLC, however, Farad (F) is used. Since the voltage of the EDLC varies simultaneously with discharge, F is used in accordance with the capacitance notation of a general condenser (capacitor). Furthermore, an ultra-high capacitance EDLC has capacitance that is one thousand to one million times larger than that of a general condenser with capacitance of mF, uF or the like. However, a conventional EDLC has an operating voltage that is significantly lower than that of an existing battery or condenser, as described above.
The energy storage amount that is a piece of data related to energy storage may be considered as a good index useful for comparison of the amount of energy even in the EDLC, in the same manner as a battery and a condenser. The energy storage amount can be obtained by the following equation:Largest energy storage amount (J)=1/2CV2 
where C is capacitance per cell (F) and V is a voltage applicable to a cell.
From the above equation, it can be seen that the largest energy storage amount is proportional to capacitance but is also proportional to the square of voltage. In order words, if the voltage increases twice for the same area, the largest energy storage amount increases by four times. If the capacitance increases twice, however, the largest energy storage amount increases twice. Accordingly, it can be considered that the best way to increase the value of the largest energy storage amount available in an EDLC is to increase the voltage.
As described above, however, in the related art, unit cells are serially connected to increase an operating voltage. This method has a problem in that a high voltage is applied to anyone of the unit cells, since voltage balance among the unit cells is broken due to repeated cycles in the capacitance of the unit cell, a capacitance change rate, ESR, a resistance change rate, a leakage current, and a leakage current change rate. This causes an electrolyte to be dissolved (the electrolyte is dissolved when a voltage of 3.0V or more is applied thereto). It also results in increased internal resistance, reduced capacitance, and the like.